WHAT IS AUDIO FORENSICS?
What is audio forensics? Forensic audio analysis is the scientific examination comparison and/or evaluation of an audio recording in legal matters. The scientific process behind audio forensic analysis must be performed in a forensic lab that is equipped with the appropriate tools and follows best practice protocols in order to process the audio recording with integrity and accuracy.
The primary aspects of audio forensics are establishing the authenticity of audio evidence, performing enhancement of audio recordings to improve speech intelligibility and the audibility of low-level sounds, and interpreting and documenting sonic evidence, such as identifying talkers, transcribing dialog, and reconstructing crime or accident scenes and timelines. Modern audio forensics makes extensive use of digital signal processing, with the former use of analog filters now being obsolete. Techniques such as adaptive filtering and discrete Fourier transforms are used extensively.
THREE CATEGORIES OF AUDIO FORENSIC SCIENTISTS:
Audio is an auditory perceptual phenomenon. Most courts use some version of the “fair and accurate representation” standard as a
measure of acceptance. The same general principles apply regardless of the format in which the audio is recorded and the
physical media onto which the recording is stored.
SWGDE Best Practices for Forensic Audio V 2.2 PG 10
WHAT TYPES OF RECORDERS CREATE DIGITAL AUDIO EVIDENCE?
DIGITAL AUDIO FORMATS
Digital audio evidence is most commonly created by passive and active recording systems. A passive recording system is a recording system that doesn’t store information in its memory system. An active recording system is a recording that stores information in its memory system. Active recording systems are most commonly produced with a digital storage medium such as a HDD, SSD or Volatile (flash) memory. Video recorders create digital video recordings in these types of formats:
ACQUISITION OF AUDIO EVIDENCE
The acquisition of digital audio recordings must be done in accordance with the acceptable protocols within the scientific community. These protocols have been established by an organization called SWGDE. The acquisition of digital audio recordings is broken up into three categories; establish/examine the chain of custody, request the original, retrieve the recordings using acceptable methods.
EXAMINE CHAIN OF CUSTODY
First, examine the chain of custody. What type of equipment was used to create the evidence? How was the evidence handled from the time of its creation to the delivery to the courtroom, as well as investigators and experts? Are there authentic chain of custody documents and reports that outline the chain of custody? Sometimes a chain of custody log from law enforcement will be included, which will strengthen the authenticity of the audio evidence. If the investigation of the chain of custody reveals inconsistencies, more often than not that recording is determined to lack authenticity and integrity.
ESTABLISH CHAIN OF CUSTODY
If the expert is able to retrieve the evidence from the original source, in most cases that will automatically create and establish an authentic chain of custody, if done properly. This retrieval process must be documented through video recording or images in order to provide an accurate record of what the expert did during the retrieval process. If it isn’t possible to retrieve the recording(s), then the forensic expert must carefully go through all of the documents and reports that arrived with the evidence. But if the chain of custody cannot be established, the forensic expert must rely on other techniques as well as their own expertise to determine the authenticity of the chain of custody, such as request of the original.
REQUEST FOR THE ORIGINAL
“As a general rule, a forensic audio laboratory should request the original recording or the earliest generation available. An original recording is the first manifestation of sound in a recoverable stored format. If the original recording is on analog media, playback and duplication rely on physical processes that introduce noise and degrade the signal, even if slightly. A copy of an analog recording can never be an exact duplicate. An original digital recording is a bit stream from which the acoustic audio signal can be generated. Exact copies of that bit stream can be made. With digital evidence, each stage of copying can be exact with no loss of quality between generations. The exactness can be tested and confirmed through the use of a hash function. Therefore, a bit-stream duplicate of a recorded file is equivalent to the original. –SWGDE Best Practices for Forensic Audio V 2.2 PG 10
“Means of securing the recorded evidence must be evaluated based on their effect on the recorded signal, and the available method of transfer preserving the evidence in a condition as close to the original as possible should be chosen. Use multiple means of collection if it is not apparent which available means will produce the highest quality. –SWGDE Best Practices for Forensic Audio V 2.2 PG 10.
Appropriate retrieval methods are as follows:
As defined in SWGDE/SWGIT Digital & Multimedia Evidence Glossary, authentication is the process of substantiating that the asserted provenance of data is true. As defined in SWGDE Best Practices for Forensic Audio, an audio authentication examination seeks to determine if a recording is consistent with the manner in which it is alleged to have been produced. The term “authentication” is often used in a legal context to describe the establishment of a proper legal foundation for the admission of a recording as evidence into a judicial proceeding. This is generally accomplished by a party involved in the events recorded or involved in the recording process affirming that the events heard during playback of the recording are consistent with that party’s recollection of the events as they transpired. When this is contested or cannot be accomplished, a scientific analysis may be conducted to test disputed claims. – Digital Audio Authentication Best Practices V 1.3 Pg. 4
Forensic audio enhancement services are the scientific process of clarifying audio recordings using safe techniques to preserve speech quality. This is crucial for the trier of fact can make determinations about the events within the recorded evidence. Above all, the goal of forensic enhancement is to better hear dialogue by removing unwanted noise. This scientific process of improving the quality of dialogue is also known as speech enhancement or voice enhancement.
Below are some of the most popular clarification and enhancement techniques that can be applied to an investigation.
Forensic enhancement and analysis tools include software and hardware that are implemented and used on a powerful desktop computer. Methods applied to this scientific examination must be executed in a systematic and accurate nature. Our audio forensics service is sought after by law enforcement agencies, attorney’s, government agencies and private clients. Opinions and conclusions drawn by the audio forensic expert MUST follow best practices. A forensic report is created on completion that exhibits all activity performed in the lab as well as forensic processes performed. That way, any other qualified audio forensic expert can follow our steps multiple times and establish the same conclusions and opinions. Our audio forensic processes and executed with the utmost accuracy as we testify in court on our opinions and conclusions.
AUDIO COMPARISON INVESTIGATION
Comparison of audio signals, speech and other acoustic events, to better understand unknown or disputed details.
EXPERT WITNESS TESTIMONY
An Audio Forensic Expert has the scientific knowledge, training, and expertise necessary to perform audio forensic analysis on recordings used in a courtroom proceeding. The expert witness has previous courtroom experience testifying, provides opinions based on objective reasoning, and helps the Trier of Fact understand the audio evidence that is offered in litigation.